1. Field of the Invention
This invention relates to a magnetic disk which can be used in a highly reliable magnetic storage device capable of recording/reproducing information of high density, and a magnetic storage device that uses the recording medium of the invention.
The demand for making high density recording media has grown as magnetic storage devices have increased in mass storage. To meet the demand, noise reduction in the media is required. For reduction in noise, it is effective to reduce the grain size of magnetic layers and to decrease Br·t, which is a product of residual magnetic flux density (Br) and the film thickness of magnetic layers (t).
In the former case, however, decrease in volume of magnetic grains gives rise to degradation of thermal stability factors (Ku·v/kT) (Ku: constant of crystalline magnetic anisotropy, v: volume of magnetic crystal grains, k: Boltzmann's constant, T: absolute temperature), which give an index for thermal stability. Thus, suppression is made possible to some degree by enhancing Ku but enhancement of Ku is limitative since the overwriting characteristic is rapidly degraded when the writing capacity of a recording head is exceeded. As for the case where Br·t is decreased, Ku·v/kT is decreases with reduction in film thickness, so thermal stability is decreased. As described above, degradation in thermal stability becomes an obstacle to reduction in noise.
Antiferromagnetically coupled media, a technique that is compatible both for improvement of thermal stability and for noise reduction, (Apple. Phys. Lett., vol. 77, pp. 2581-2583, October (2000), appl. Phys. Lett., vol. 77, pp. 3806-3808, December (2000)), has been proposed in recent years. According to this technique, magnetic layers are formed in a double-layered structure, in which antiferromagnetic coupling is made through Ru intermediate layers. In antiferromagnetically coupled media, magnetization of lower magnetic layers on a side of a substrate and magnetization of upper magnetic layers on a side of a protective film are antiparallel in a state of residual magnetization.
Therefore, Br·t of an antiferromagnetically coupled medium is generally represented by Br·t=Br2·t2−Br1·t1 with the use of Br·t (Br2·t2) of upper magnetic layers and Br·t (Br1·t1) of lower magnetic layers. Accordingly, Br·t can be set low while magnetic layers are maintained at a thick film thickness, as compared to a medium composed of magnetic monolayers.
As described above, antiferromagnetically coupled media afford reduction in noise to some degree, but reduction in Br·t alone provides insufficient noise reduction and inplane magnetic recording density of 50 megabits/mm2 or more cannot be attained. In addition, there is not yet enough knowledge with respect to such factors as layer configuration, magnetic characteristics of Co alloy materials used for magnetic layers, and so on have on recording/reproducing characteristics, so that the structure has not yet been made optimal in realizing reduction in noise.